A part of the background hereof lies in the development of heterojunction bipolar transistors which operate as light-emitting transistors and transistor lasers. Reference can be made for example, to U.S. Pat. Nos. 7,091,082, 7,286,583, 7,354,780, 7,535,034 and 7,693,195; U.S. Patent Application Publication Numbers US2005/0040432, US2005/0054172, US2008/0240173, US2009/0134939, and US2010/0034228; and to PCT International Patent Publication Numbers WO/2005/020287 and WO/2006/093883. Reference can also be made to the following publications: Light-Emitting Transistor: Light Emission From InGaP/GaAs Heterojunction Bipolar Transistors, M. Feng, N. Holonyak, Jr., and W. Hafez, Appl. Phys. Lett. 84, 151 (2004); Quantum-Well-Base Heterojunction Bipolar Light-Emitting Transistor, M. Feng, N. Holonyak, Jr., and R. Chan, Appl. Phys. Lett. 84, 1952 (2004); Type-II GaAsSb/InP Heterojunction Bipolar Light-Emitting Transistor, M. Feng, N. Holonyak, Jr., B. Chu-Kung, G. Walter, and R. Chan, Appl. Phys. Lett. 84, 4792 (2004); Laser Operation Of A Heterojunction Bipolar Light-Emitting Transistor, G. Walter, N. Holonyak, Jr., M. Feng, and R. Chan, Appl. Phys. Lett. 85, 4768 (2004); Microwave Operation And Modulation Of A Transistor Laser, R. Chan, M. Feng, N. Holonyak, Jr., and G. Walter, Appl. Phys. Lett. 86, 131114 (2005); Room Temperature Continuous Wave Operation Of A Heterojunction Bipolar Transistor Laser, M. Feng, N. Holonyak, Jr., G. Walter, and R. Chan, Appl. Phys. Lett. 87, 131103 (2005); Visible Spectrum Light-Emitting Transistors, F. Dixon, R. Chan, G. Walter, N. Holonyak, Jr., M. Feng, X. B. Zhang, J. H. Ryou, and R. D. Dupuis, Appl. Phys. Lett. 88, 012108 (2006); The Transistor Laser, N. Holonyak and M Feng, Spectrum, IEEE Volume 43, Issue 2, February 2006; Signal Mixing In A Multiple Input Transistor Laser Near Threshold, M. Feng, N. Holonyak, Jr., R. Chan, A. James, and G. Walter, Appl. Phys. Lett. 88, 063509 (2006); and Collector Current Map Of Gain And Stimulated Recombination On The Base Quantum Well Transitions Of A Transistor Laser, R. Chan, N. Holonyak, Jr., A. James, and G. Walter, Appl. Phys. Lett. 88, 14508 (2006); Collector Breakdown In The Heterojunction Bipolar Transistor Laser, G. Walter, A. James, N. Holonyak, Jr., M. Feng, and R. Chan, Appl. Phys. Lett. 88, 232105 (2006); High-Speed (/spl ges/1 GHz) Electrical And Optical Adding, Mixing, And Processing Of Square-Wave Signals With A Transistor Laser, M. Feng, N. Holonyak, Jr., R. Chan, A. James, and G. Walter, Photonics Technology Letters, IEEE Volume: 18 Issue: 11 (2006); Graded-Base InGaN/GaN Heterojunction Bipolar Light-Emitting Transistors, B. F. Chu-Kung et al., Appl. Phys. Lett. 89, 082108 (2006); Carrier Lifetime And Modulation Bandwidth Of A Quantum Well AlGaAs/InGaP/GaAs/InGaAs Transistor Laser, M. Feng, N. Holonyak, Jr., A. James, K. Cimino, G. Walter, and R. Chan, Appl. Phys. Lett. 89, 113504 (2006); Chirp In A Transistor Laser, Franz-Keldysh Reduction of The Linewidth Enhancement, G. Walter, A. James, N. Holonyak, Jr., and M. Feng, Appl. Phys. Lett. 90, 091109 (2007); Photon-Assisted Breakdown, Negative Resistance, And Switching In A Quantum-Well Transistor Laser, A. James, G. Walter, M. Feng, and N. Holonyak, Jr., Appl. Phys. Lett. 90, 152109 (2007); Franz-Keldysh Photon-Assisted Voltage-Operated Switching of a Transistor Laser, A. James, N. Holonyak, M. Feng, and G. Walter, Photonics Technology Letters, IEEE Volume: 19 Issue: 9 (2007); Experimental Determination Of The Effective Minority Carrier Lifetime In The Operation Of A Quantum-Well n-p-n Heterojunction Bipolar Light-Emitting Transistor Of Varying Base Quantum-Well Design And Doping; H. W. Then, M. Feng, N. Holonyak, Jr., and C. H. Wu, Appl. Phys. Lett. 91, 033505 (2007); Charge Control Analysis Of Transistor Laser Operation, M. Feng, N. Holonyak, Jr., H. W. Then, and G. Walter, Appl. Phys. Lett. 91, 053501 (2007); Optical Bandwidth Enhancement By Operation And Modulation Of The First Excited State Of A Transistor Laser, H. W. Then, M. Feng, and N. Holonyak, Jr., Appl. Phys. Lett. 91, 183505 (2007); Modulation Of High Current Gain (β>49) Light-Emitting InGaN/GaN Heterojunction Bipolar Transistors, B. F. Chu-Kung, C. H. Wu, G. Walter, M. Feng, N. Holonyak, Jr., T. Chung, J.-H. Ryou, and R. D. Dupuis, Appl. Phys. Lett. 91, 232114 (2007); Collector Characteristics And The Differential Optical Gain Of A Quantum-Well Transistor Laser, H. W. Then, G. Walter, M. Feng, and N. Holonyak, Jr., Appl. Phys. Lett. 91, 243508 (2007); Transistor Laser With Emission Wavelength at 1544 nm, F. Dixon, M. Feng, N. Holonyak, Jr., Yong Huang, X. B. Zhang, J. H. Ryou, and R. D. Dupuis, Appl. Phys. Lett. 93, 021111 (2008); and Optical Bandwidth Enhancement Of Heterojunction Bipolar Transistor Laser Operation With An Auxiliary Base Signal, H. W. Then, G. Walter, M. Feng, and N. Holonyak, Jr. Appl. Phys. Lett. 93, 163504 (2008).
FIGS. 1 and 2 illustrate an example of an existing tilted charge light emitter; that is, a light-emitting transistor (“LET”) as described in the above referenced patent documents and publications. An n+ GaAs subcollector region 105 has an n-type GaAs collector region 110 deposited thereon, followed by a p+ AlGaAs/GaAs base region 120, having an n-type InGaAs quantum well (QW) 126. An emitter mesa is deposited over the base, and includes n-type InGaP emitter layer 130, and n-type AlGaAs aperture layer 140, and an n+ GaAs cladding layer 150. Lateral oxidation can be conventionally used to obtain annular oxide 141 and form the central aperture. The collector electrode or contact metallization is shown at 107, the base contact metallization is shown at 122, and the emitter contact metallization is shown at 152. FIG. 2 shows a plan view of the FIG. 1 metallizations; that is, opposing collector contacts (common connection not shown), the base contact 122 including an outer annular ring, and the emitter contact 152 including an inner annular ring.
FIG. 1 also has arrows that illustrate the flow of electron current and hole current in typical light-emitting transistor operation. As described, for example, in the above referenced documents, light-emitting transistors, transistor lasers, and certain two terminal light emitters are sometimes referred to as “tilted charge” devices, owing to the “tilted” base charge distribution (as could be illustrated on the device band diagram) which locks the base electron-hole recombination in “competition” with the charge “collection” at the reverse-biased collector junction, thus selecting (“filtering”) and allowing only “fast” recombination in the base (assisted by the quantum well(s)) at an effective lifetime of the order of picoseconds. [Reference can be made, for example, to the above-listed documents, including reference to a two-terminal tilted charge light emitter disclosed in U.S. Patent Application Publication No. US2010/0034228, and reference can also be made to U.S. patent application Ser. No. 12/655,806, filed Jan. 7, 2010, which is a parent application hereof.]
In existing tilted charge devices, the optical cavity or window, defined in part by an aperture formed with an oxide, is placed after the base and emitter contact. Due to the high base sheet resistant and large current gain (emitter current) of the tilted charge device, the voltage difference across the base emitter junction is the greatest along the edge defined by the oxide aperture. This forces the recombination events (which result in the desired optical output) to localize along the perimeter of the oxide aperture, as current injection is largest in the region where voltage difference is largest. The junction voltage decreases towards the center of the optical cavity. This phenomenon is represented in FIGS. 1 and 2, and can be further understood from the modeling of device operation as shown in the simplified circuit model of FIG. 3. In FIG. 3, the regions and contacts correspond with those of like reference numerals in FIG. 1. In the model, 307, 320, and 330 respectively represent the collector, base, and emitter resistances, 308 represents collector current components, and 340 represents the spatial components of base/emitter voltage. As was first seen in FIG. 1, the path of least resistance for electron conduction is along the edge defined by the oxide aperture. In the model of FIG. 3, this results in V4 being substantially greater than V3, and V1 being substantially greater than V2. This causes most of the recombination events to localize nearer the edge of the base layer, and less recombination at and near the center of the base layer (see sketch of light output representation of FIG. 2).
FIG. 4 is a graph showing detected optical output of the device (as detector photo current in μA) as a function of device base current (in mA). The optical output for larger emitter diameter devices saturates at larger base current input. Saturation of light is attributed to quantum well saturation.
In FIG. 5, the optical output density and emitter current density for different emitter sizes (and hence, aperture sizes) is conveniently normalized to the aperture perimeter “area” (shaded area of inset in FIG. 5.) The area is determined by assuming a constant shallow penetration into the optical cavity. The result indicates that recombination is localized along the edge of the device. Maximum light output is therefore determined by the active perimeter defined by the oxide aperture rather than the area of the total optical cavity.
FIG. 6 illustrates pulsed current measurement for various emitter sizes showing light output for both 10% and 50% pulsed current measurements to be substantially the same. Results indicate that light saturation for the device was not caused by heating but by localized quantum well saturation.
FIG. 7 is a top view photograph of the type of existing device of FIG. 1, wherein the collector (C), base (B), and emitter (E) metallizations are denoted, and the optical cavity or window is indicated by an arrow. The light-emitting transistor of the Figure has a 10 um emitter mesa and aperture defined optical cavity of 6 um. The optical cavity is located after the base and emitter contacts (i.e., above them, as in FIG. 1). The active perimeter of this device is therefore about 18 μm. Similarly, FIG. 8 shows an existing tilted charge light-emitting diode (see, for example, copending U.S. patent application Ser. No. 12/655,806, filed Jan. 7, 2010, assigned to the same assignees as the present Application), wherein the emitter (E) and base/drain (BD) metallizations are denoted and, again, the device has a 10 um emitter mesa and aperture defined optical cavity of 6 um. The optical cavity is again located after the base and emitter contacts. Again, the active perimeter of this device is about 18 μm.
In the described types of devices, as above noted, the optical window or cavity is placed after the base and emitter contact. Due to the high base sheet resistant and large current gain (emitter current) of the tilted charge device, the voltage difference across the base emitter junction is greatest along the edge defined by the oxide aperture. As explained above, this forces the recombination events (which result in the desired optical output) to localize along the perimeter of the oxide aperture, as current injection is largest in the region where voltage difference is largest. The junction voltage decreases towards the center of the optical cavity, with attendant disadvantages.
Among the objects of the present invention are to overcome these and other limitations of existing light-emitting devices, such as the described tilted-charge light emitters, and to improve light emission of light-emitting and lasing semiconductor devices.